There are various industrial processes which require exposing objects to a controlled non-ambient temperature while simultaneously maintaining a specific composition of the gas to which the objects are exposed while in the controlled-temperature zone. Commonly the controlled gas composition is oxygen-deficient so as to prevent oxidation or corrosion at the controlled temperature, particularly if the controlled temperature is hot. In many such processes there must be a continuous movement of objects into or out of the controlled-temperature zone, typically on a conveyor. This requires the presence of one or more openings (ports) in the boundary of the controlled-temperature chamber. Together, these features create conflicting requirements whereby the openings (ports) and the temperature difference between the chamber and the external atmosphere encourage motion of gas and inleakage of the external atmosphere, but the inleakage of the external atmosphere works against the requirements for controlled composition of the gas inside the chamber.
Perhaps the best example of such a process is ovens used in the electronics industry to attach electronic components to printed circuit boards. A common type of oven is a solder reflow oven, in which electronic components are placed in precise position on circuit boards (Printed Wiring Boards) along with unmelted solder, and the circuit board is moved through an oven of appropriate temperature distribution to melt the solder and cause attachment of the components to the circuit board. Some solder reflow ovens require an inert atmosphere to prevent oxidation of the solder. A similar situation occurs in wave soldering apparatus, in which a pool of liquid solder must be maintained for extended periods of time with a very clean surface. In those soldering operations for which inerting is necessary, typically the inert gas used to create the oxygen-deficient atmosphere is nitrogen, for economic reasons. A typical desired maximum oxygen concentration is 25 parts per million. In order to maintain the concentration of oxygen in the oven at desired low levels, a continuous flow of inertant is fed into the oven and a corresponding outflow of gas occurs at the ports of the oven with that gas outflow essentially pushing back oxygen which would otherwise enter at the oven ports. The chief reason why this is necessary is that the hot nitrogen inside the oven is less dense than the ambient-temperature air outside, and this density difference causes a buoyancy-driven flow pattern in the vicinity of the ports of the oven. This buoyancy-driven flow pattern promotes mixing of oxygen-containing air with the inert gas and can actually bring a distinct flowpath of oxygen-containing air into the oven. For ovens which operate for long periods of time, which have appreciable areas of entrance and exit opening, and which have significant purity requirements for the gas inside them, the operating expense due to consumption of nitrogen can be significant, e.g., tens of thousands of dollars per year.